The catalytic activity of niobium oxide for hydrogen absorption is determined using an optical indicator technique to measure hydrogen sorption kinetics. The catalytic activity of NbOx is Studied as a function of the oxygen concentration determined from electron spectroscopy. STM measurements display a clustered NbOx, Surface, disproving a diffusion-control led kinetics. The catalytic effect results from dissociation of hydrogen by niobium oxide. The measured activation energy depends strongly on the oxygen concentration and reaches a minimum of 0.6 eV at the highest oxygen content (x = 2.5, i.e., Nb2O5). Pure Nb does not display a catalytic effect. Oxide Surfaces with a high concentration of artificial oxygen vacancies are less catalytically active. (c) 2005 Elsevier Inc. All rights reserved.